The present invention is related to copy protection processes such as those which cause a darkening or lightening effect in an illegal copy of a video signal when displayed, and more particularly to a copy protection process which enhances the copy protection effect by adding further chroma distortion to an illegal copy of the copy protected video signal.
It follows therefore that a preferred embodiment of the invention is used in combination with typical copy protection processes which selectively attenuate the video signal. Such video copy protection signals typically may consist of sync or pseudo sync and automatic gain control (AGC) pulses. In a video cassette recorder (VCR), erroneous gain levels are generated by the sync and/or pseudo sync/AGC signals, which cause the VCR to turn down the video gain. Once the video gain is turned down, a number of enhancement signals may be added primarily to affect the integrity of the sync signal in the VCR and/or television (TV) set. These typical copy protection signals cause the VCR and TV set to sense a false sync signal due at least in part to the lowered video gain. Such signals will for instance cause the TV set to scan an illegal copy of the video signal improperly by either pre scanning and/or post scanning the video horizontal lines. Also, the VCR or TV set may develop extra or erroneous color burst sample pulses in an illegal copy.
Such typical copy protection signals thus may also contain various signals to enhance the copy protection effect such as end of line signals, end of field signals, post pseudo sync and pre pseudo sync signals, etc. Descriptions of the above mentioned signal attenuating copy protection processes are found in, for example, U.S. Pat. No. 4,631,603 issued Dec. 23, 1986 to Ryan, U.S. Pat. No. 4,918,098 issued Apr. 4, 1989 to Ryan, and U.S. Pat. No. 4,163,253 issued Jul. 31, 1979 to Morio et al., all incorporated by reference herein.
It is common practice for color videotape recorders to utilize the color burst component of a video signal to generate velocity error-correction signals to compensate for the inevitable fluctuations of the head to tape velocity. The velocity error corrector circuit in a video-tape recorder measures the phase of the color burst signal on a line-by-line basis. Any variation of phase is assumed to be due to head to tape velocity errors, and a correction signal is generated. The correction signal alters the phase of the chrominance signal in an attempt to eliminate these errors.
In accordance with a copy protection technique disclosed in U.S. Pat. No. 4,577,216 ('216) issued Mar. 18, 1986 to Ryan and known as the “Colorstripe™” copy protection process or system, (and incorporated by reference herein), deliberate phase errors are introduced into the color burst signal by phase modulating the color burst signal with a noise signal to produce “Colorstripe bursts” of incorrect phase.
The velocity error correction circuit in the videotape recorder (VCR) accordingly will attempt to eliminate the deliberate phase errors introduced. In so doing, the velocity error correction circuit will transfer the error from the color burst to the chrominance signal, thus giving rise to color noise or error in the viewed picture reproduced from the videotape recording. However, the characteristics of the modified color burst signal are such that the phase modulation is ignored by a television receiver, i.e., TV set, which produces a substantially normal color picture.
The term Colorstripe is a trademark of Macrovision Corporation, Sunnyvale, Calif. However, to facilitate the description, a variation of the term is used herein after, namely, “color stripe”, or “color stripe burst” to indicate a correspondingly modified normal color burst.
The color stripe process has a disadvantage in that the color stripe signal cannot be recorded onto a video tape without causing a “playability” problem when the recorded tape is viewed upon reproduction. That is, the color stripe signal can be used successfully in a video signal transmission environment such as cable, broadcast television or as an add on signal to a digital video disk (DVD) player, but cannot be included in a prerecorded videotape environment without degrading the playability of the prerecorded tape. “Playability” is an expression indicative of the (degree of success of) viewing of a video signal containing an anti-copy protection (ACP) process when the protected video signal is applied directly to a TV set.
Accordingly, it would be highly desirable to provide a color burst modifying copy protection process which could be used in a prerecorded videotape environment as well as in the signal transmission and DVD player environments.